Coelom or true cavity generally refers to a large fluid-filled space lying between outer body wall and
inner digestive tube.
It arises as a secondary cavity between 2 layers of embryonic mesoderm and contains most of the
viscera organs, and is bounded on all sides by a definite coelomic epithelium.
| Primary Coelom | Secondary Coelom |
| Also called a False Coelom or Pseudocoelom Derived from the Blastocoel of the embryo Internal organs remain free in it, since it is not bound by peritoneum or mesoderm Enclosed by ectoderm and endoderm e.g. Rotifers, Roundworms etc. | Blastocoel is obliterated by embryonic archenteron, without forming a primary coelom True coelom develops within the embryonic mesoderm and is lined by a layer of flattened mesodermal epithelial cells True coelom appeared for the first time in Annelids Present in all higher Bilateria |
Coelomic Division of Metazoa
| Acoelomata | No body cavity or coelom Embryonic mesoderm is a solid layer, space between endoderm and ectoderm is filled with mesenchyme and muscle fibres. e.g. Porifera, Coelenterata, Ctenophora, Platyhelminthes, Nemertinea |
| Pseudocoelomata | Persistent blastocoel enclosed between outer ectoderm and inner ectoderm, and not lined by mesoderm e.g. Acanthocephala, Ectoprocta, Aschelminthes |
| Coelomata | Body space is a true coelom, enclosed by mesoderm on both sides Bilateria from Annelida to Echinodermata Types of Coelom: 1) Schizocoelomata: Coelom arises by splitting of endomesodermal bands which originate from blasts-oral region of larva and extend between ectoderm and mesoderm. Annelida, Arthropoda, Mollusca 2) Mesenchymal Coelomata: Mesenchymal cells rearrange to form a coelom. Regarded as an aberrant schizocoel Only in Phoronida 3) Enterocoelomata: Arise in form of mesodermal pouches from larval archenteron. After separation from endoderm, the pouches fuse and expand until they touch the gut and body wall. Deuterostomes (Echinodermata, Hemichordata, Chordata), Brachiopoda |
Significance of Coelom
Progressive development of complex structure: Permits greater size and contributes directly to
development of excretory, reproductive and muscular systems
Surrounds the internal organs like a water jacket and protects them from external shocks.
Flexibility to the body
Coelomic fluid functions as a hydraulic skeleton
Circulatory medium for the transport and distribution of nutritive substances and gases
Excretory substances are collected into Coelomic fluid and then passed out of the body through
Nephridia
Ova and Sperms are extruded through gonoducts connecting coelom to the exterior